1. Field of the Invention
The present invention relates to a surface-mount quartz crystal oscillator, and more particularly to a surface-mount crystal oscillator which is of a small height and a low cost.
2. Description of the Related Art
Since surface-mount crystal oscillators are small in size and weight, they are widely used as a frequency or time reference source in portable electronic devices. In recent years, efforts are being made to achieve more size reductions in surface-mount crystal oscillators, and there are demands for reductions to be accomplished in the cost of surface-mount crystal oscillators through increased productivity. In the application to cellular phone devices or the like, the surface-mount crystal oscillators are required to be small in height as well as in planar outer dimensions.
FIG. 1 shows in cross section a conventional surface-mount crystal oscillator.
As shown in FIG. 1, the conventional surface-mount crystal oscillator generally comprises IC (Integrated Circuit) chip 2 and quartz crystal blank 3 which are housed in casing 1. Casing 1 is basically made of three-layer laminated ceramics and has a recess. Steps are formed in the inner surface of a frame wall which defines the recess. A pair of connecting terminals 7 is disposed on the upper surface of one of the steps for electrically connecting to crystal blank 3. IC connecting terminals 6 are disposed on the bottom of the recess in casing 1 for connection to IC chip 2. Connecting terminals 7 and IC connecting terminals 6 are electrically connected to each other by conductive paths (not shown) disposed in casing 1.
IC chip 2 comprises an integrated oscillating circuit connected to crystal blank 3 which functions as a quartz crystal unit. A plurality of IC terminals 4 for connecting IC chip 2 to crystal blank 3 and an external circuit are mounted on at least one of principal surfaces of IC chip 2. IC terminals 4 and IC connecting terminals 6 are electrically and mechanically connected to each other by ultrasonic thermo-compression bonding with bumps 5, for example, interposed therebetween, thus fixing one of the principal surfaces of IC chip 2 to the bottom of the recess in casing 1.
Crystal blank 3 comprises a substantially rectangular AT-cut quartz crystal blank, for example. Excitation electrodes are formed respectively on both principal surfaces of crystal blank 3, and extension electrodes extend from the respective excitation electrodes to opposite sides of one end of crystal blank 3. The sides of the end of crystal blank 3 to which the extension electrodes extend are fixed to the step of casing 1 on which connecting electrodes 7 are disposed, thereby holding crystal blank 3 horizontally in casing 1. Crystal blank 3 is fixed to the step by electrically conductive adhesive 10. Therefore, IC chip 2 is electrically connected to crystal blank 3 by IC terminals 4, IC connecting terminals 6, and connecting terminals 7.
The conventional surface-mount crystal oscillator is assembled as follows: IC chip 2 is fixedly mounted in casing 1, and crystal blank 3 is also fixedly mounted in casing 1. Thereafter, cover 8 is placed on the upper surface of the frame wall that surround the recess in casing 1, thereby hermetically sealing IC chip 2 and crystal blank 3 in casing 1. Mounting terminals 9 that are electrically connected to IC connecting terminals 6 are disposed on the outer surface of the bottom of casing 1. Mounting terminals 9 are used to install the surface-mount crystal oscillator on a wiring board and connect the surface-mount crystal oscillator to an external circuit.
The surface-mount crystal oscillator described just above is assembled by fixing IC chip 2 to the bottom of the recess in casing 1 and thereafter holding the opposite sides of one end of crystal blank 3 on the step in the recess in casing 1. After crystal blank 3 is held in position, it is measured for electric characteristics. If the measured electric characteristics do not meet predetermined criteria, then the assembly is rejected as a defective product. Since the rejected assembly includes expensive IC chip 2, the productivity is lowered and the cost is increased.
It is difficult to reduce the height of the surface-mount crystal oscillator because casing 1 is of a laminated ceramics structure with two stages and cover 8 is placed thereon.
A quartz crystal unit changes its vibrating characteristics and electric characteristics when it is fixed in place because conditions under which the crystal blank vibrates are different before and after the crystal blank is fixed in place. Therefore, even if crystal blank 3 has good vibrating characteristics or electric characteristics, e.g., crystal impedance or frequency vs. temperature characteristics, by itself, the characteristics may possibly become poor after the crystal blank is fixed to the step, and the whole assembly may be rejected as failing to meet the criteria.
Japanese laid-open patent publications Nos. 2001-308644 (JP, P2001-308644A) and 6-232631 (JP, 6-232631, A) disclose surface-mount crystal oscillators which are assembled as follows: An assembling process in which a crystal blank is fixed to the bottom of a recess defined in a casing, and then measured for vibrating characteristics. If the measured vibrating characteristics fail to satisfy predetermined criteria, the assembly is rejected as a defective product with no IC chip included. An IC chip is installed only in the casing only when the installed crystal blank is judged as having accepted vibrating characteristics. However, there is a limitation on efforts to reduce the height of the disclosed surface-mount crystal oscillators because a separate cover needs to be mounted on the casing.